This invention relates to an idling rpm feedback control system for internal combustion engines, and more particularly to an idling rpm feedback control system which uses an on-off type valve for control of the quantity of supplementary air and is adapted to supply supplementary air to the engine in synchronism with a signal indicative of a top dead center of the engine.
An idling rpm feedback control system is conventionally known which is adapted to set the engine rpm at a desired value of engine rpm depending upon a load on the engine at engine idle, detect the difference between the desired engine rpm and actual engine rpm and control the supply of supplementary air to the engine in response to the detected difference so as to make the same difference zero, to thereby maintain the actual engine rpm at the desired value.
In such an idling rpm feedback control system as disclosed in the above publication, it is also known to use an on-off type control valve for control of the quantity of supplementary air and to control the valve by varying the pulse duty cycle of a drive signal therefor. Some idling rpm feedback control systems of this kind are combined with fuel injection systems which are adapted to detect the quantity of suction air being supplied to the engine and electronically control the fuel injection quantity in response to the detected suction air quantity. According to such fuel injection systems, if a parameter representative of the suction air quantity such as intake pipe pressure largely fluctuates, it is difficult to detect the value of the parameter with accuracy and therefore also determine a correct value of the required suction air quantity, making it difficult to supply a proper quantity of fuel to the engine.
To overcome this disadvantage, a method has been proposed which comprises setting the pulse duty cycle of the drive signal for the supplementary air quantity control valve at a frequency value higher than the cycle of fluctuations of the pressure of the suction air or a like parameter so as to reduce the flucutuations of the suction air pressure to be caused by the introduction of supplementary air into the intake pipe. However, according to this proposed method, the frequency at which the supplementary air quantity control valve is opened and closed is rather high, which necessitates designing the valve so as to impart sufficient endurance thereto.